A process known as carburizing is widely employed to harden surfaces of steel gears, steel shafts, and other components to impart higher hardness, toughness, and wear resistance thereto. For example, steel vehicle transmission components typically are carburized to impart a higher hardness, toughness, and wear resistance thereto.
A typical carburizing process for vehicle transmission steel gear components involves machining a hot-forged, cold-forged or cold-rolled soft low carbon steel or alloy steel to provide close tolerance machined components, heating the machined components in a batch furnace to a temperature of 1550 to 1850 degrees F. in a gaseous carburizing atmosphere present in the furnace for a time to diffuse a predetermined amount of carbon into the surface of the component to form a carbon-enriched surface case on an underlying core of the component. After the components have been maintained at the required carburizing temperature and time, the components are quickly cooled to a lower temperature by quenching into a liquid quenchant, such as oil or water, to promote metallurgical phase changes in the component. Such metallurgical phase changes in low carbon steel components and low alloy steel components typically include the formation of a martensitic surface case on a banitic core of the component upon quenching. For a representative steel transmission component, a traditional carburizing treatment is conducted for approximately 6 hours from start to finish to produce a hardened surface case of approximately 0.025 inch depth on the component. The carburized component is quenched in oil to a temperature of 150-250 degrees F. Following quenching, the component typically is tempered in well known manner to form a tempered martensitic surface case on a banitic core of the gear component.
A problem oftentimes experienced with such carburizing treatments involves distortion of certain distortion-sensitive machined steel components, such as components having a large diameter with thin walls and/or having non-uniform/non-symmetric cross-sections, when they are quenched from the carburizing temperature to the much lower quench temperature (e.g. rapidly cooling the components from 1550 degrees F. to 250 degrees F. by oil quenching). This distortion is due to high internal stresses created by quenching the component and causes many of the quenched components to fall outside of the predetermined dimensional tolerances established for an acceptable component.
Another problem experienced with such carburizing treatments involves internal oxidation of certain steels (e.g. alloy steels) having easily oxidized alloying elements, such as Cr, where discrete oxide regions or networks are formed inside the component during the carburization treatment that can adversely affect mechanical properties of the component in service.
An object of the present invention is to provide a carburizing method that reduces the start-to-finish time to produce a carburized component.
Another object of the present invention is to provide a carburizing method that reduces distortion and internal oxidation of the components.